That isn't true. Altcoins have great time when Bitcoin's dominance weans. It's obvious that when more people buy and trade Bitcoin they neglect altcoins. Thus dominance for Bitcoin increases and that of altcoins crashes. For instance, right now Bitcoin controls 64.9% of the total market cap and this won't give altcoins air to rise. But once the dominance lowers to say below 60%, go check the prices of altcoins and see how great they would be.